In a system for reading/writing information from a magnetic disk, a magneto-resistive read/write head is used to detect magnetic information stored in substantially circular tracks on the magnetic disk. In order for the system to successfully read or record magnetic information from/to a given track on the magnetic disk, the read/write head must be precisely positioned and centered over that track. The read/write head is typically mounted on a radially-movable arm that is controlled by a disk servo control system.
The servo control system receives, from the magnetic disk surface via the read/write head, servo information signals that indicate the position of the read/write head on the magnetic disk. Conventionally, servo information is pre-recorded on the magnetic disk in the form of high-frequency magnetic flux transitions. The pattern of these magnetic flux transitions defines a binary value that typically corresponds to the track number. Accordingly, the pattern of the servo information communicated to the servo control system, via the read servo signals, identifies the specific track over which the read/write head is traveling.
The amplitude of servo information signals indicates the position of the read/write head with respect to the center of the track. With one type of magnetic media, when the amplitude of servo signals (i.e., the signal read by the read/write head when passing over the track and representing the amplitude of the servo information) is equal to zero, that indicates the read/write head is centered over the track. With another type of magnetic media, the magnetic servo information is located on either side of the track. With this type of media, an amplitude difference of zero also indicates that the read/write head is centrally positioned over the track. Conversely, a negative or positive amplitude difference corresponds to an offset between the position of the read/write head and the center of the track. In either case, once modulated, the servo signal is referred to as Position Error Signal (PES) because it feeds back to the servo control system the error associated with the position of the read/write head. Additionally, servo signals having a positive or negative amplitude indicate the extent of the displacement of the read/write head. Thus, by obtaining PES feedback, the servo control system can generate corrective control signals to adjust the position of the read/write head over the center of the track.
There have been proposed methods and circuitries for modulating servo signals into Position Error Signals. These prior art methods and circuitries typically involve the employment of an integrating capacitor for the purpose of measuring the amplitude of the analog servo signals.
For example, one prior proposed solution is shown in FIG. 1. The circuitry of FIG. 1 includes a Variable Gain Amplifier (VGA) 110, a multiplier 120, an Automatic Gain Control (AGC) circuit 140, a timing acquisition circuit 130, a charge Pump (CP) 150, an integrating capacitor 170, and an analog-to-digital converter (A/D) 180. Initially, switch 160 is opened so that no charge is accumulating in the integrating capacitor 170. Switches 133 and 147 are closed so that the AGC circuit 140 and the timing acquisition circuit 130 are in operation. Switch 190 is also closed so that capacitor 170 is not being charged. Switches 160, 133, and 147 remain in their initial state for a predetermined settlement period of time that is sufficient to (i) set the gain of the variable gain amplifier through AGC 140, and to (ii) obtain the proper timing from the timing acquisition circuit 130. During this settlement period, the VGA 110 gain is adjusted by inputting the output of VGA 110 into clipper 142 and into multiplier 144. The output of clipper 142 is then multiplied with the VGA output via multiplier 144. The current output of multiplier 144 then charges capacitor 148. The resulting voltage across capacitor 148 adjusts the gain of VGA 110. The adjustment continues for a period of time until the current output of multiplier 144 is equal to a pre-defined value of a constant current source 146. Once that equilibrium is achieved there will be no current going into capacitor 148 and, therefore, no further adjustment to the gain of VGA 110.
Additionally, during this settlement time, the timing acquisition circuit 130 locks in the phase of the output of the VGA 110 to obtain the system timing signal. This is done by qualifying the output of VGA 110 via an Automatic Pulse Qualifier (APQ) 132 and locking the phase of the output of APQ 132 via Phase Locked Loop (PLL) 134. The output timing signal is delayed for the remainder of the servo signal cycle, via Delay 136, before it is inputted into multiplier 120.
Once the predetermined settlement period expires, the VGA gain is deemed to have stabilized and the timing signal's locked phase is deemed appropriate for the timing of the system. At this time, switches 147 and 133 are opened to fix the gain of VGA and the locked phase of the timing signal. Switch 160 is then closed and switch 190 is opened to allow the charging of the integrating capacitor 170. The output of VGA is inputted into multiplier 120 and multiplied by the timing signal that is outputted from the timing acquisition circuit 130. Thereafter, the output of multiplier 120 is converted into a current signal via CP 150 and allowed to charge the integrating capacitor 170. The resulting voltage across the integrating capacitor 170 is representative of the amplitude of the servo information signal. After the integrating capacitor 170 is charged switch 160 is opened and the capacitor 170 voltage is then converted into a digital 10-bit PES via A/D 180 through a successive analog-to-digital conversion operation.
It is readily apparent to one skilled in the art that the design parameters for the prior art components (namely multiplier 144, multiplier 120, current source 146 and integrating capacitor 170) must be matched exactly in order for the above-described prior art system to obtain accurate representations of the PES value. In addition, the preset settlement period must be accurately determined in order for the system to yield an accurate result. Moreover, the total time of (i) the settlement period, (ii) the time needed for the integrating capacitor 170 to charge, and (iii) the time needed for the successive 10-bit analog-to digital conversion, introduces appreciable delay in the prior art servo control system.
What is desired is a simple system that can yield an accurate PES value without the drawbacks of the prior art design. More specifically, what is needed is a faster and more accurate system that can yield a PES value without the added delay of the prior art.